1. Field of the Invention
The present invention relates to a method of preparing of nickel-metal hydroxycarbonate having a high density, and particularly, to a method of preparing of nickel-metal hydroxycarbonate having a high density, which can be used for preparing a cell having a high capacity and a long lifetime.
2. Description of the Related Art
Recently, with the trend to miniaturize and lighten the potable electronic machines such as camera-VTR integrated systems, audio systems and lap-top personal computers and portable phones and the like, there is a need to improve the efficiency and capacity of a cell which is used as a power source for these machines. In particular, it is desirable to lower the production cost in economical aspects.
In general, cells are classified as follows: a primary cell, such as manganese battery, an alkali battery, a mercury battery and a silver oxide battery, which are not recharged electrically and hence are discarded after discharging; a secondary cell, such as a lead storage battery, a Nil-MH (nickel-metalhydride) battery using metal hydride as a negative active material, sealed nickel-cadmium battery, a lithium-metal battery, lithium ion battery (LIB), a lithium-polymer battery (LPB), which can be recharged electrically to their original condition after use; a fuel battery; and a solar battery.
The primary cell has a disadvantage in that the cell has a low capacity, a short lifetime and is not reused. On the other hand, the secondary cell has an advantage in that the cell has a long lifetime by recharging and reusing, and has a higher voltage than a primary cell so that the cell has a high performance and efficiency, and the cell can be reused.
Among the secondary cells described above, a nickel-based cell is desirable in environmental aspects because of the highly developed recycling technology of Ni. And the capacity of an electrode plate is increased by packing amount per volume by packing an active material of paste into a multi-porous plate and alkali-resistant plate to provide a cell having a high capacity, and it is broadly used now.
Nowadays, nickel hydroxide is used as a negative active material in a nickel-based cell, and the charge-discharge reaction of the material is as follows: EQU .beta.--Ni(OH).sub.2 .revreaction..beta.--NiOOH
During the reversible reaction, the oxidation number of nickel changes by one (Ni(II).revreaction.Ni(III)). Therefore, the theoretical capacity of a cell produced by using nickel hydroxide is 289 mAh/g. However, the oxidation number of nickel changes from +2.3 to +3.0.about.3.7 in a real charge-discharge reaction (in redox reaction of nickel), thereby the practical capacity of the nickel-based cell varies from 200 to 400 mAh/g (70 to 140% of the theoretical value).
Regardless of above-described advantages, a high oxidation number of nickel decreases the lifetime of a cell and an electrode, causes severe self-discharge, and causes low reversibility of reaction. Therefore, the actual available capacity varies from 250 to 280 mAh/g. In a positive electrode of a nickel cell, the major reason for the electrode inferiority is due to swelling of the electrode from the expansion of the electrode volume, which happens when nickel hydroxide changes from .beta.--NiOOH to low density .gamma.--NiOOH. The swelling of the electrode causes separation of an active material, decrease of conductivity, and severe decrease of the lifetime and efficiency of the electrode. The low density .gamma.--NiOOH is formed due to the compact crystalline structure of .beta.-nickel hydroxide having a high density. The compact crystalline structure results in the decrease of the number of internal micropores. Therefore, hydrogen ions can not move smoothly in the crystalline structure. Therefore, it is necessary to prevent the formation of low density .gamma.--NiOOH having a low density in the .beta.--Ni(OH).sub.2 .revreaction..beta.--NiOOH reversible reaction, in order to improve the characteristics of an electrode.
A new material, nickel-metal hydroxycarbonate which is prepared by adding elements such as cobalt, cardmium, zinc, et al. to nickel hydroxide to substitute a part of the nickel with the element to maintain a stable .alpha. form in a strong alkali electrolyte, thereby preventing the change of .beta.--NiOOH to .gamma.--NiOOH. The new material, Ni-M hydroxycarbonate is used in an .alpha.--NiOOH to .gamma.--NiOOH reversible reaction The method allows the transformation of a lattice by substituting the nickel, thereby facilitating the movement of hydrogen ions to decrease the overvoltage. Therefore, the method can effectively prevent the formation of .gamma.--NiOOH having low density from .beta.--NiOOH. Furthermore, a method in which the conductivity of an active material is improved by using cobalt-based oxide or other adding agents which forms an effective network in a strong alkali solution, is widely used together with the above-mentioned method.
Conventionally, a method of preparing the above nickel-metal compound is as follows: first, Ni salt is reacted with ammonia and sodium hydroxide solution to prepare Ni-Zn hydroxide. Second, Ni-Al hydroxycarbonate is prepared by forming precipitates in the form of a colloid at a pH of 8 to 11, as shown below in Reaction 1. The colloid is dried and treated to have the powder form.
Reaction 1! EQU 8NiSO.sub.4 +Al.sub.2 (SO.sub.4).sub.3 +22NaOH.fwdarw.Ni.sub.8 Al.sub.2 (OH).sub.22 +11Na.sub.2 SO.sub.4 EQU Ni.sub.8 Al.sub.2 (OH).sub.22 +Na.sub.2 CO.sub.3 .fwdarw.Ni.sub.8 Al.sub.2 (OH).sub.20 CO.sub.3 +2NaOH
The material is used for a reversible reaction of .alpha.--Ni(OH).sub.2 .revreaction..gamma.--NiOOH, which has a small change of density. Furthermore, the oxidation number of nickel changes by a large amount during the reaction, that is, the number of electrons exchanged is increased. Therefore, in theory a cell prepared by using the material has a high capacity so that it can be expected to remarkably increase capacity. Furthermore, swelling of an electrode can be prevented by using the material, and therefore the lifetime of the cell can be improved. However, practically, because Ni-Al hydroxycarbonate prepared by the method has an irregular shape in the form of a colloid, the material has a low density and its shape is irregular, and it is difficult to increase the density of the material and to make it a globular shape. Therefore, when the material is used as an active material, it is very difficult to apply it to a cell because of the low density and irregular shape of the material.